Semiconductor device

ABSTRACT

A semiconductor device includes: a first semiconductor layer formed on a substrate and formed of a nitride-based semiconductor; a second semiconductor layer formed on a surface of the first semiconductor layer and formed of a nitride-based semiconductor having a wider band-gap than the first semiconductor layer; first and second electrodes formed on a surface of the second semiconductor layer; an inter-electrode insulator film that is formed between the first and second electrodes on the surface of the second semiconductor layer; and a dielectric constant adjustment layer formed on the inter-electrode insulator film and formed of an electric insulator. The first electrode has a field plate portion formed so as to ride on the inter-electrode insulator film, and the dielectric constant adjustment layer has a first layer that contacts a lateral end portion of the field plate portion and a second layer formed on the first layer.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and incorporates by referencethe entire contents of Japanese Patent Application No. 2012-238429 filedin Japan on Oct. 29, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a semiconductor device.

2. Description of the Related Art

Wide band-gap semiconductors have high breakdown voltage, goodelectronic transport property, and good thermal conductivity, and thusare very appealing as a material for high temperature, high power, orhigh frequency semiconductor devices. A typical wide band-gapsemiconductor is a nitride-based semiconductor, which is made of GaN,AlN, InN, BN, or a mixed crystal of at least two of GaN, AlN, InN, andBN. Further, in a semiconductor device that has an AlGaN/GaNhetero-junction structure, for example, two-dimensional electron gas isgenerated at an hetero-junction interface by the piezoelectric effect.The two-dimensional electron gas has high electron mobility and highcarrier density. Therefore, semiconductor devices having such anAlGaN/GaN hetero-junction structure, like Schottky barrier diodes orfield effect transistors, for example, have high voltage endurance, lowon-resistance, and fast switching speed, and are very suitable for powerswitching applications.

Further, a device having an AlGaN/GaN hetero-junction structure isdisclosed, in which a Schottky electrode rides on a surface protectionfilm that is formed on a surface of a semiconductor layer and formed ofan electric insulator, and forms a field plate structure, in order torealize higher voltage endurance (refer to “N. Zhang, U. K. Mishra,“High Breakdown GaN HEMT with Overlapping Gate Structure”, IEEE ElectronDevice Letters, vol. 21, no. 9, 2000”).

In a semiconductor device having a hetero-junction structure, in orderto reduce its on-resistance or suppress occurrence of current collapse,a carrier density of its two-dimensional electron gas is preferablyincreased. If the carrier density of the two-dimensional electron gas isincreased, however, intense electrical field concentration tends tooccur in the device when a reverse voltage is applied to the device(e.g., in a Schottky barrier diode, when a reverse voltage is appliedbetween an anode electrode and a cathode electrode). This causesreduction of voltage endurance and current collapse of the device.

Accordingly, there is a need to provide a semiconductor device in whichreduction of on-resistance or suppression of current collapse isachieved and reduction of voltage endurance is prevented.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a semiconductor deviceincludes: a first semiconductor layer that is formed on a substrate andformed of a nitride-based semiconductor; a second semiconductor layerthat is formed on a surface of the first semiconductor layer and formedof a nitride-based semiconductor having a wider band-gap than the firstsemiconductor layer; a first electrode that is formed on a surface ofthe second semiconductor layer; a second electrode that is formed on thesurface of the second semiconductor layer; an inter-electrode insulatorfilm that is formed between the first electrode and the second electrodeon the surface of the second semiconductor layer; and a dielectricconstant adjustment layer that is formed on the inter-electrodeinsulator film and formed of an electric insulator. The first electrodehas a field plate portion formed so as to ride on the inter-electrodeinsulator film, and the dielectric constant adjustment layer has a firstdielectric constant adjustment layer that contacts a lateral end portionof the field plate portion and a second dielectric constant adjustmentlayer formed on the first dielectric constant adjustment layer.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor deviceaccording to a first embodiment;

FIG. 2 is a diagram illustrating an example of electric field intensitydistributions generated in the semiconductor device illustrated in FIG.1;

FIG. 3 is a diagram illustrating an example of leakage currentproperties of semiconductor devices according to a first example and asecond example;

FIG. 4 is a diagram illustrating a relation between dielectric constantsand breakdown voltages of first dielectric constant adjustment layersaccording to examples of the present invention;

FIG. 5 is a table listing numerical values of data points illustrated inFIG. 4;

FIG. 6 is a diagram of an example of current collapse properties of thesemiconductor devices according to the first and second examples;

FIG. 7 is a schematic cross-sectional view of a semiconductor deviceaccording to a second embodiment;

FIG. 8 is a schematic cross-sectional view of a semiconductor deviceaccording to a third embodiment; and

FIG. 9 is a schematic cross-sectional view of a semiconductor deviceaccording to a fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of a semiconductor device according to thepresent invention will be described in detail with reference to thedrawings. The present invention is not limited by the followingembodiments. In the drawings, the same or corresponding elements aredesignated by the same reference numerals or symbols as appropriate.Furthermore, it is to be noted that the drawings are schematic, andrelations among dimensions of the elements may differ from the actual.Portions having relations or ratios among their dimensions that differamong the drawings may be included.

First Embodiment

FIG. 1 is a schematic cross-sectional view of a semiconductor deviceaccording to a first embodiment of the present invention. Thissemiconductor device 100 is a Schottky barrier diode and includes: afirst semiconductor layer 2; a second semiconductor layer 3; a firstelectrode 4; a second electrode 5; an inter-electrode insulator film 6;wiring metals 7 and 8; and a dielectric constant adjustment layer 9,which are formed on a substrate 1.

The substrate 1 is a base layer for the first semiconductor layer 2 and,for example, has a structure in which a desired semiconductor layer,such as a buffer layer as appropriate, is formed on a substrate made ofSi, SiC, sapphire, GaN, or the like. Accordingly, a desired layer may beinterposed between the substrate and the first semiconductor layer 2.

The first semiconductor layer 2 is a layer formed of a nitride-basedsemiconductor and functions as an electron transit layer. The secondsemiconductor layer 3 is formed on a surface of the first semiconductorlayer 2, is formed of a nitride-based semiconductor having a widerband-gap than the first semiconductor layer 2, and functions as anelectron supply layer. For example, the first semiconductor layer 2 ismade of GaN and the second semiconductor layer 3 is made of AlGaN, butthe nitride-based semiconductor materials forming the firstsemiconductor layer 2 and the second semiconductor layer 3 are notparticularly limited, as long as their band-gaps satisfy a desiredrelation.

A layer thickness of the second semiconductor layer 3 is 1 nm to 50 nm,for example, and preferably 20 nm to 25 nm. Further, an Al compositionof the second semiconductor layer 3 is 25%, for example, but may be 10%to 50%, and more preferably 20% to 35%.

The inter-electrode insulator film 6 is formed between the firstelectrode 4 and the second electrode 5 on the surface of the secondsemiconductor layer 3, and is formed of an electric insulator made ofSiN, SiO₂, or Al₂O₃, for example.

The first electrode 4 is formed on the surface of the secondsemiconductor layer 3. The first electrode 4 forms a Schottky contactwith the second semiconductor layer 3 and has a field plate portion 4 aformed so as to ride on the inter-electrode insulator film 6 on thesecond semiconductor layer 3. The first electrode 4 has a Ni/Austructure, for example. This first electrode 4 functions as an anodeelectrode. The second electrode 5 is formed on the surface of the secondsemiconductor layer 3 with the inter-electrode insulator film 6 betweenthe first electrode 4 and the second electrode 5, and forms an ohmiccontact with the second semiconductor layer 3. The second electrode 5has a Ti/Al structure, for example. This second electrode 5 functions asa cathode electrode.

The wiring metal 7 is formed on the first electrode 4. The wiring metal8 is formed on the second electrode 5. The wiring metals 7 and 8 aremade of a metal for wiring, such as Al or Au.

The dielectric constant adjustment layer 9 is formed on theinter-electrode insulator film 6 and formed of an electric insulator.The dielectric constant adjustment layer 9 has: a first dielectricconstant adjustment layer 9 a facing the inter-electrode insulator film6 and contacting a lateral end portion 4 aa of the field plate portion 4a of the first electrode 4; and a second dielectric constant adjustmentlayer 9 b formed on the first dielectric constant adjustment layer 9 a.

The second semiconductor layer 3 has a wider band-gap than the firstsemiconductor layer 2, and thus two-dimensional electron gas 2 a as acarrier is generated at an interface between the first semiconductorlayer 2 and the second semiconductor layer 3 due to the piezoelectriceffect. A carrier density Ns of the two-dimensional electron gas 2 a isof the order of 1×10¹² cm⁻² to 2×10¹³ cm⁻², for example.

As described above, for the on-resistance and the current collapseproperty between the anode electrode (the first electrode 4) and thecathode electrode (the second electrode 5) upon application of apositive voltage between the anode electrode and the cathode electrode,the carrier density of the two-dimensional electron gas 2 a ispreferably high, and preferably set to 1×10¹³ cm⁻², for example.

However, when a reverse voltage is applied between the anode electrodeand the cathode electrode, the two-dimensional electron gas 2 a isgradually depleted from the first electrode 4 side. When this happens,electrical field is concentrated to the lateral end portion 4 aa of thefield plate portion 4 a of the first electrode 4, and because the higherthe carrier density of the two-dimensional electron gas 2 a is, thehigher the intensity of the concentrated field becomes, the voltageendurance is lowered.

In this respect, in the semiconductor device 100, because the firstdielectric constant adjustment layer 9 a formed of an electric insulatoris in contact with the lateral end portion 4 aa of the field plateportion 4 a, the depletion of the two-dimensional electron gas 2 abecomes easier to extend along the first dielectric constant adjustmentlayer 9 a. This mitigates the concentration of electric field to thelateral end portion 4 aa. As a result, in the semiconductor device 100,the carrier density of the two-dimensional electron gas 2 a is increasedto be able to achieve reduction of on-resistance or suppression ofcurrent collapse, and to prevent reduction of voltage endurance.Further, the first dielectric constant adjustment layer 9 a is locatedabove the two-dimensional electron gas 2 a and has an effect of pullingout the two-dimensional electron gas 2 a, which is a carrier, fromabove, when a reverse voltage is applied. This makes the two-dimensionalelectron gas 2 a depleted before the electric field at the lateral endportion 4 aa become too high. Accordingly, the reduction of voltageendurance in the semiconductor device 100 is further prevented.

For such effects of facilitating the extending of depletion and ofpulling out the carrier to be demonstrated, a higher dielectric constantof the first dielectric constant adjustment layer 9 a is preferable. Todescribe with reference to FIG. 1, arrows L1, L2, and L3 in FIG. 1represent paths of pulling out the carrier from the two-dimensionalelectron gas 2 a and will be called path 1, path 2, and path 3,respectively. The path 1 represents a path of pulling out from a bottomsurface of the first electrode 4, the path 2 represents a path ofpulling out from the lateral end portion 4 aa of the field plate portion4 a of the first electrode 4 through the first dielectric constantadjustment layer 9 a and the inter-electrode insulator film 6, and thepath 3 represents a path of pulling out from the wiring metal 7 throughthe second dielectric constant adjustment layer 9 b, the firstdielectric constant adjustment layer 9 a, and the inter-electrodeinsulator film 6. An electric charge pulled out through the path 2 isdetermined by a capacity at an interface between the lateral end portion4 aa of the field plate portion 4 a of the first electrode 4 and thefirst dielectric constant adjustment layer 9 a, a capacity at aninterface between the lateral end portion 4 aa of the field plateportion 4 a of the first electrode 4 and the inter-electrode insulatorfilm 6, a capacity of the inter-electrode insulator film 6, and thelike, and thus to increase the effect of pulling out the carrier, ahigher dielectric constant of the first dielectric constant adjustmentlayer 9 a is preferable. Further, the effect of pulling out the electriccharge via the path 3 is small because a distance between the wiringmetal 7 and the two-dimensional electron gas 2 a is large.

FIG. 2 is a diagram of an example of electric field intensitydistributions generated in the semiconductor device illustrated inFIG. 1. A graph illustrated below the semiconductor device 100 is agraph that illustrates, by enlarging in a width direction, the electricfield intensity distributions in the vicinity of the lateral endportions 4 aa of the field plate portions 4 a upon application of areverse voltage. When a dielectric constant ∈_(r)1 of the firstdielectric constant adjustment layer 9 a is changed to 1, 3, 5, 7, and9, the peak value of the electric field intensity decreases asillustrated in the graph. Accordingly, the higher the dielectricconstant of the first dielectric constant adjustment layer 9 a, the morethe concentration of electric field to the lateral end portion 4 aa ismitigated. Further, FIG. 2 illustrates values of dielectric breakdownfields when the inter-electrode insulator film 6 is made of SiO₂. FromFIG. 2, the dielectric constant ∈_(r)1 of the first dielectric constantadjustment layer 9 a is preferably three or larger, so that the electricfield intensity in the inter-electrode insulator film 6 in the vicinityof the lateral end portion 4 aa becomes smaller than the dielectricbreakdown field of the inter-electrode insulator film 6.

The first dielectric constant adjustment layer 9 a contacts the lateralend portion 4 aa of the field plate portion 4 a of the first electrode4, and a layer thickness of the first dielectric constant adjustmentlayer 9 a is about the same as the field plate portion 4 a. The layerthickness of the first dielectric constant adjustment layer 9 a isseveral hundred nanometers, for example. A length of the field plateportion 4 a is 4 μm, for example.

The first dielectric constant adjustment layer 9 a may be formed of amaterial, which has been adjusted to have a desired dielectric constantand includes any one of or any combination as appropriate of: a materialthat includes at least one of SiO₂, Si₃N₄, Al₂O₃, and MgO; a materialthat includes a polyimide film in which ceramic particles having a highdielectric constant have been mixed, the ceramic particles including atleast one of TiO₂, SiO₂, Al₂O₃, HfO, ZrO, HfSi_(x)O_(y), andZrSi_(x)O_(y); a material that includes a polymeric material includingat least one metal of Cu, Ni, Ag, Al, Zn, Co, Fe, and Mn; a materialthat includes a silicone resin material; and a material that includes ahigh-dielectric polymeric material.

The second dielectric constant adjustment layer 9 b may be formed of amaterial having the same dielectric constant as the first dielectricconstant adjustment layer 9 a. In particular, if the first dielectricconstant adjustment layer 9 a and the second dielectric constantadjustment layer 9 b are made of the same material, the dielectricconstant adjustment layer 9 may be regarded as being substantially of asingle-layer structure. In that case, if the dielectric constant of thesecond dielectric constant adjustment layer 9 b is ∈_(r)2,∈_(r)1=∈_(r)2.

However, if the second dielectric constant adjustment layer 9 b is tohave a function of insulating between the wiring metal 7 and the wiringmetal 8, a layer thickness of the second dielectric constant adjustmentlayer 9 b is several micrometers. When the second dielectric constantadjustment layer 9 b is as thick as this, if the dielectric constant ishigh, a parasitic capacitance of the semiconductor device 100 isincreased. Therefore, in order to achieve the semiconductor device 100having a high-speed switching property, if the dielectric constant ofthe second dielectric constant adjustment layer 9 b is ∈_(r)2,preferably ∈_(r)2<9 and ∈_(r)1>∈_(r)2. Accordingly, a relationalexpression ∈_(r)1≧∈_(r)2 is held between ∈_(r)1 and ∈_(r)2.

If ∈_(r)2 is too low, when a reverse voltage is applied, the insulationis too high and electrical discharge between the wiring metals mayoccur. In order to prevent this, preferably 1.5<∈_(r)2.

In a more specific example, the field plate portion 4 a of the firstelectrode 4 has a layer thickness of 350 nm, and the first dielectricconstant adjustment layer 9 a has about the same layer thickness. Theinter-electrode insulator film 6 is made of SiO₂ having a dielectricconstant of 3.8. The first dielectric constant adjustment layer 9 a ismade of a polyimide having a dielectric constant ∈_(r)1 of 3.5. Thesecond dielectric constant adjustment layer 9 b is made of a polyimidehaving a dielectric constant ∈_(r)2 of 3.0.

In another specific example, the field plate portion 4 a of the firstelectrode 4 has a layer thickness of 350 nm, and the first dielectricconstant adjustment layer 9 a has about the same layer thickness. Theinter-electrode insulator film 6 is made of SiO₂ having a dielectricconstant of 3.8. The first dielectric constant adjustment layer 9 a ismade of SiO₂ having a dielectric constant ∈_(r)1 of 3.8. The seconddielectric constant adjustment layer 9 b is made of a polyimide having adielectric constant ∈_(r)2 of 3.0.

In still another specific example, the field plate portion 4 a of thefirst electrode 4 has a layer thickness of 350 nm, and the firstdielectric constant adjustment layer 9 a has about the same layerthickness. The inter-electrode insulator film 6 is made of SiO₂ having adielectric constant of 3.8. The first dielectric constant adjustmentlayer 9 a is made of Si₃N₄ having a dielectric constant ∈_(r)1 of 7.9.The second dielectric constant adjustment layer 9 b is made of apolyimide having a dielectric constant ∈_(r)2 of 3.0.

A semiconductor device having the structure illustrated in FIG. 1 wasfabricated as an example of the present invention, and its voltageendurance and current collapse property were measured. As for propertiesof the fabricated semiconductor device, the first semiconductor layerwas a GaN layer having a layer thickness of 700 nm; the secondsemiconductor layer was an Al_(0.25)Ga_(0.75)N layer having a layerthickness of 30 nm; the inter-electrode insulator film was a SiO₂ filmhaving a film thickness of 600 nm; the field plate portion had a lengthof 4 μm and a thickness of 350 nm; the first dielectric constantadjustment layer was a polyimide having a layer thickness of 350 nm; thewiring metal had a thickness of 5 μm; and the second dielectric constantadjustment layer had a layer thickness of 6 μm. A distance between theanode electrode and the cathode electrode (Lac) was 10 μm.

FIG. 3 is a diagram illustrating leakage current properties ofsemiconductor devices according to a first example and a second example.In these first and second examples, dielectric constants of their firstdielectric constant adjustment layers were set to 1.9 and 3.5,respectively. In FIG. 3, the horizontal axis represents reverse voltageVr and the vertical axis represents leakage current Ir under the reversevoltage. A value of the reverse voltage when the leakage currentincreases sharply, is determined as a breakdown voltage herein.

Leakage current properties of semiconductor devices having variousdielectric constant values of their first dielectric constant adjustmentlayers were measured and their breakdown voltages were measured thereby.FIG. 4 is a diagram illustrating a relation between the dielectricconstants and the breakdown voltages of the first dielectric constantadjustment layers according to examples of the present invention. FIG. 5is a table listing numerical values of data points illustrated in FIG.4. As illustrated in FIGS. 4 and 5, it was confirmed that the larger thedielectric constant of the first dielectric constant adjustment layer,the higher the breakdown voltage.

Table 1 below illustrates simulation results for ratios of electriccharges pulled out through the path 2 to electric charges pulled outthrough the path 1 illustrated in FIG. 1, when the inter-electrodeinsulator film 6 is made of SiO₂ (its dielectric constant being 3.8),the dielectric constant ∈_(r)1 of the first dielectric constantadjustment layer 9 a is changed to 1, 3, 5, 7, and 9, and a reversevoltage of 400 V is applied. The carrier density Ns of thetwo-dimensional electron gas 2 a is 1×10¹³ cm⁻². As illustrated in Table1, the larger the dielectric constant ∈_(r)1 of the first dielectricconstant adjustment layer 9 a, the more the ratio of the electriccharges pulled out tends to be saturated, and the dielectric constant∈_(r)1 of the first dielectric constant adjustment layer 9 a ispreferably three or larger so that the ratio of the electric chargespulled out becomes 65% or larger. The dielectric constant ∈_(r)1 ispreferably nine or smaller so that formation of the first dielectricconstant adjustment layer 9 a becomes easy.

TABLE 1 Ratio of electric charges pulled εr1 out (path 2/path 1) 1 55% 365% 5 70% 7 73% 9 75%

Next, current collapse properties of the semiconductor devices of thefirst example and the second example were measured. FIG. 6 is a diagramillustrating an example of the current collapse properties of thesemiconductor devices according to the first and second examples. InFIG. 6, the horizontal axis represents reverse voltage Vr and thevertical axis represents collapse. The collapse is an amount defined byRa/Rb, when an on-resistance of the semiconductor device before areverse voltage is applied is Rb and an on-resistance of thesemiconductor device after the reverse voltage is applied is Ra.Therefore, the larger the value of the collapse, the more the currentproperty is deteriorated. As illustrated in FIG. 6, it was confirmedthat the larger the dielectric constant of the first dielectric constantadjustment layer, the more the current collapse property improved.

Second Embodiment

FIG. 7 is a schematic cross-sectional view of a semiconductor deviceaccording to a second embodiment of the present invention. Asillustrated in FIG. 7, this semiconductor device 100A has a structure inwhich the dielectric constant adjustment layer 9 in the semiconductordevice 100 according to the first embodiment illustrated in FIG. 1 issubstituted with a dielectric constant adjustment layer 9A.

The dielectric constant adjustment layer 9A is formed on theinter-electrode insulator film 6 and formed of an electric insulator.The dielectric constant adjustment layer 9A has: a first dielectricconstant adjustment layer 9Aa that faces the inter-electrode insulatorfilm 6 and contacts the lateral end portion 4 aa of the field plateportion 4 a of the first electrode 4; and a second dielectric constantadjustment layer 9Ab formed on the first dielectric constant adjustmentlayer 9Aa.

Materials forming the first dielectric constant adjustment layer 9Aa andthe second dielectric constant adjustment layer 9Ab, and preferablevalues of their dielectric constants, layer thicknesses, and the likemay be the same as their corresponding first dielectric constantadjustment layer 9 a or second dielectric constant adjustment layer 9 b.While the first dielectric constant adjustment layer 9 a of thesemiconductor device 100 is formed entirely on the inter-electrodeinsulator film 6 up to the second electrode 5, the first dielectricconstant adjustment layer 9Aa of the semiconductor device 100A is formedpartially on the inter-electrode insulator film 6 from the lateral endportion 4 aa of the field plate portion 4 a toward the second electrode(cathode electrode) 5. A part of the second dielectric constantadjustment layer 9Ab covers a surface of the inter-electrode insulatorfilm 6 on the second electrode 5 side.

Also in this semiconductor device 100A, like the semiconductor device100, the first dielectric constant adjustment layer 9Aa demonstrates theeffect of facilitating the extending of the depletion and the effect ofpulling out the carrier. As a result, the carrier density of thetwo-dimensional electron gas 2 a is able to be increased to reduce theon-resistance or suppress the current collapse, and to prevent thereduction of the voltage endurance.

In a specific example, the field plate portion 4 a of the firstelectrode 4 has a layer thickness of 350 nm, and the first dielectricconstant adjustment layer 9Aa has a layer thickness of about the same.The inter-electrode insulator film 6 is made of SiO₂ having a dielectricconstant of 3.8. The first dielectric constant adjustment layer 9Aa ismade of Si₃N₄ having a dielectric constant ∈_(r)1 of 7.9. The seconddielectric constant adjustment layer 9Ab is made of a polyimide having adielectric constant ∈_(r)2 of 3.0. An amount of the carrier pulled outfrom the lateral end portion 4 aa of the field plate portion 4 a of thefirst electrode 4 changes depending on a capacity between the lateralend portion 4 aa and a top surface of the inter-electrode insulator film6. Therefore, when the first dielectric constant adjustment layer 9Aamade of Si₃N₄ having the dielectric constant ∈_(r)1 of a large value isformed in the vicinity of the lateral end portion 4 aa and the seconddielectric constant adjustment layer 9Ab made of a polyimide having thedielectric constant ∈_(r)2 of a relatively small value is formed in theother region, a capacity between the wiring metals 7 and 8 is able to befurther reduced as compared with an example in which the firstdielectric constant adjustment layer 9Aa made of Si₃N₄ is formed on theentire surface.

Third Embodiment

FIG. 8 is a schematic cross-sectional view of a semiconductor deviceaccording to a third embodiment of the present invention. Thissemiconductor device 200 is a high electron mobility transistor (HEMT),and includes the first semiconductor layer 2, the second semiconductorlayer 3, a first electrode 24, a second electrode 25, a third electrode10, inter-electrode insulation films 26 a and 26 b, wiring metals 27 and28, and a dielectric constant adjustment layer 29, which are formed onthe substrate 1.

The inter-electrode insulator films 26 a and 26 b are respectivelyformed between the first electrode 24, the second electrode 25, and thethird electrode 10, on a surface of the second semiconductor layer 3,and are formed of an electric insulator such as SiN, SiO₂, or Al₂O₃, forexample.

The first electrode 24 forms a Schottky contact with the secondsemiconductor layer 3 and has field plate portions 24 a and 24 b formedso as to ride on the inter-electrode insulator films 26 a and 26 b onthe second semiconductor layer 3. The first electrode 24 has a Ni/Austructure, for example. This first electrode 24 functions as a gateelectrode.

The second electrode 25 is formed on the surface of the secondsemiconductor layer 3 with the inter-electrode insulator film 26 ainterposed between the first electrode 24 and the second electrode 25,and forms an ohmic contact with the second semiconductor layer 3. Thesecond electrode 25 has a Ti/Al structure, for example. The secondelectrode 25 functions as a drain electrode. The third electrode 10 isformed on the surface of the second semiconductor layer 3 with theinter-electrode insulator film 26 b interposed between the firstelectrode 24 and the third electrode 10 and forms an ohmic contact withthe second semiconductor layer 3. The third electrode 10 has a Ti/Alstructure, for example. This third electrode 10 functions as a sourceelectrode.

The wiring metal 27 is formed on the third electrode 10. The wiringmetal 28 is formed on the second electrode 25. The wiring metals 27 and28 are made of a metal for wiring, such as Al or Au.

The dielectric constant adjustment layer 29 is formed on theinter-electrode insulation films 26 a and 26 b and is formed of anelectric insulator. Further, the dielectric constant adjustment layer 29has: a first dielectric constant adjustment layer 29 a that contacts alateral end portion 24 aa of the field plate portion 24 a of the firstelectrode 24; a first dielectric constant adjustment layer 29 b thatcontacts a lateral end portion 24 ba of the field plate portion 24 b;and a second dielectric constant adjustment layer 29 c formed on thefirst dielectric constant adjustment layers 29 a and 29 b.

In this semiconductor device 200, like in the semiconductor device 100,because the first dielectric constant adjustment layer 29 a formed of anelectric insulator contacts the lateral end portion 24 aa of the fieldplate portion 24 a, depletion of the two-dimensional electron gas 2 atends to extend along the first dielectric constant adjustment layer 29a when a reverse voltage is applied between the source electrode (thethird electrode 10) and the drain electrode (the second electrode 25),and thus concentration of electric field to the lateral end portion 24aa is mitigated. In addition, the effect of pulling out the carrier bythe first dielectric constant adjustment layer 29 a is alsodemonstrated. As a result, in the semiconductor device 200, a carrierdensity of the two-dimensional electron gas 2 a is able to be increasedto achieve the reduction of on-resistance or the suppression of currentcollapse, and the reduction of voltage endurance is able to beprevented.

Materials forming the first dielectric constant adjustment layers 29 aand 29 b and the second dielectric constant adjustment layer 29 c, andpreferable values of dielectric constants, layer thicknesses, and thelike are the same as their corresponding first dielectric constantadjustment layer 9 a or the second dielectric constant adjustment layer9 b in the semiconductor device 100.

Fourth Embodiment

FIG. 9 is a schematic cross-sectional view of a semiconductor deviceaccording to a fourth embodiment of the present invention. Asillustrated in FIG. 9, this semiconductor device 200A has a structure inwhich the dielectric constant adjustment layer 29 in the semiconductordevice 200 according to the third embodiment illustrated in FIG. 8 issubstituted with a dielectric constant adjustment layer 29A.

The dielectric constant adjustment layer 29A is formed on theinter-electrode insulation films 26 a and 26 b, and formed of anelectric insulator. The dielectric constant adjustment layer 29A has: afirst dielectric constant adjustment layer 29Aa that contacts thelateral end portion 24 aa of the field plate portion 24 a of the firstelectrode 24; a first dielectric constant adjustment layer 29Ab thatcontacts the lateral end portion 24 ba of the field plate portion 24 b;and a second dielectric constant adjustment layer 29Ac formed on thefirst dielectric constant adjustment layers 29Aa and 29Ab.

Materials forming the first dielectric constant adjustment layers 29Aaand 29Ab, and the second dielectric constant adjustment layer 29Ac, andpreferable values of dielectric constants, layer thicknesses, and thelike may be the same as their corresponding first dielectric constantadjustment layer 29 a or 29 b, or the second dielectric constantadjustment layer 29 c. Further, the first dielectric constant adjustmentlayer 29Aa is formed partially on the inter-electrode insulator film 26a from the lateral end portion 24 aa of the field plate portion 24 atoward the second electrode 25. Accordingly, a part of the seconddielectric constant adjustment layer 29Ac covers a surface of theinter-electrode insulator film 26 a on the second electrode 25 side.

Also in this semiconductor device 200A, like in the semiconductor device200, the effect of facilitating the extending of the depletion and theeffect of pulling out the carrier are demonstrated by the firstdielectric constant adjustment layer 29Aa. As a result, a carrierdensity of the two-dimensional electron gas 2 a is able to be increasedto achieve the reduction of the on-resistance or the suppression of thecurrent collapse, and the reduction of the voltage endurance is able tobe prevented.

In a specific example, the field plate portions 24 a and 24 b of thefirst electrode 24 have a layer thickness of 350 nm, and the firstdielectric constant adjustment layers 29Aa and 29Ab have a layerthickness of about the same. The inter-electrode insulation films 26 aand 26 b are made of SiO₂ having a dielectric constant of 3.8. The firstdielectric constant adjustment layers 29Aa and 29Ab are made of Si₃N₄having a dielectric constant ∈_(r)1 of 7.9. The second dielectricconstant adjustment layer 29Ac is made of a polyimide having adielectric constant ∈_(r)2 of 3.0. An amount of the carrier pulled outfrom the lateral end portion 24 aa of the field plate portion 24 a ofthe first electrode 24 varies depending on a capacity between thelateral end portion 24 aa and a top surface of the inter-electrodeinsulator film 26 a. Therefore, by forming the first dielectric constantadjustment layer 29Aa made of Si₃N4 having a dielectric constant ∈_(r)1of a large value in the vicinity of the lateral end portion 24 aa andforming the second dielectric constant adjustment layer 29Ac made of apolyimide having a dielectric constant ∈_(r)2 of a relatively smallvalue in the other region on the second electrode 25 side, the effect ofbeing able to reduce the capacity between the wiring metals 27 and 28 isobtained as compared with an example in which the first dielectricconstant adjustment layer 29Aa made of Si₃N₄ is formed on the entiresurface.

The semiconductor devices according to the first and second embodimentsare Schottky barrier diodes, while the semiconductor devices accordingto the third and fourth embodiments are high electron mobilitytransistors (HEMT). However, the present invention is applicable toother various types of semiconductor devices. For example, thesemiconductor device 200 according to the third embodiment may bestructured as a metal insulator semiconductor (MIS) HEMT type fieldeffect transistor by interposing a gate insulation film between thefirst electrode 24 and the second semiconductor layer 3.

In the embodiments described above, the material referred to as SiN orSiO₂ represents a group generally referred to as SiN or SiO₂. That is,for example, the material described as SiN or SiO₂ includes SiN_(x) orSiON.

According to an embodiment of the present invention, a semiconductordevice is realized, by which reduction of on-resistance or suppressionof current collapse is achieved and reduction of voltage endurance isprevented.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A semiconductor device, comprising: a firstsemiconductor layer that is formed on a substrate and formed of anitride-based semiconductor; a second semiconductor layer that is formedon a surface of the first semiconductor layer and formed of anitride-based semiconductor having a wider band-gap than the firstsemiconductor layer; a first electrode that is formed on a surface ofthe second semiconductor layer; a second electrode that is formed on thesurface of the second semiconductor layer; an inter-electrode insulatorfilm that is formed between the first electrode and the second electrodeon the surface of the second semiconductor layer; and a dielectricconstant adjustment layer that is formed on the inter-electrodeinsulator film and formed of an electric insulator, wherein the firstelectrode has a field plate portion formed so as to ride on theinter-electrode insulator film, and the dielectric constant adjustmentlayer has a first dielectric constant adjustment layer that contacts alateral end portion of the field plate portion and a second dielectricconstant adjustment layer formed on the first dielectric constantadjustment layer, wherein when a dielectric constant of the firstdielectric constant adjustment layer is ∈_(r)1 and a dielectric constantof the second dielectric constant adjustment layer is∈_(r)2:1.5<∈_(r)2<9; and ∈_(r)1≧∈_(r)2.
 2. The semiconductor deviceaccording to claim 1, wherein a ratio of an electric charge of a carrierpulled out from the lateral end portion of the field plate portion ofthe first electrode through the inter-electrode insulator film and thefirst dielectric constant adjustment layer, to an electric charge of acarrier pulled out from a bottom surface of the first electrode is 65%or larger.
 3. The semiconductor device according to claim 1, wherein adielectric constant ∈_(r)1 of the first dielectric constant adjustmentlayer satisfies a relation of 3≦∈_(r)1≦9.
 4. The semiconductor deviceaccording to claim 1, wherein the first dielectric constant adjustmentlayer includes at least one of SiO₂, Si₃N₄, Al₂O₃, and MgO.
 5. Thesemiconductor device according to claim 1, wherein the first dielectricconstant adjustment layer includes a polyimide film in which ceramicparticles of at least one of TiO₂, SiO₂, Al₂O₃, HfO, ZrO, HfSi_(x)O_(y),and ZrSi_(x)O_(y) are mixed.
 6. The semiconductor device according toclaim 1, wherein the first dielectric constant adjustment layer includesa polymeric material including at least one metal of Cu, Ni, Ag, Al, Zn,Co, Fe, and Mn.
 7. The semiconductor device according to claim 1,wherein the first dielectric constant adjustment layer includes asilicone resin material.
 8. The semiconductor device according to claim1, wherein the first dielectric constant adjustment layer includes ahigh-dielectric polymeric material.
 9. The semiconductor deviceaccording to claim 1, wherein the first electrode is an anode electrode,the second electrode is a cathode electrode, and the semiconductordevice is a diode.
 10. The semiconductor device according to claim 1,wherein the first electrode is a gate electrode, the second electrode isa drain electrode, and the semiconductor device is a high electronmobility transistor.
 11. A semiconductor device, comprising: a firstsemiconductor layer that is formed on a substrate and formed of anitride-based semiconductor; a second semiconductor layer that is formedon a surface of the first semiconductor layer and formed of anitride-based semiconductor having a wider band-gap than the firstsemiconductor layer; a first electrode that is formed on a surface ofthe second semiconductor layer; a second electrode that is formed on thesurface of the second semiconductor layer; an inter-electrode insulatorfilm that is formed between the first electrode and the second electrodeon the surface of the second semiconductor layer; and a dielectricconstant adjustment layer that is formed on the inter-electrodeinsulator film and formed of an electric insulator, wherein the firstelectrode has a field plate portion formed so as to ride on theinter-electrode insulator film, and the dielectric constant adjustmentlayer has a first dielectric constant adjustment layer that contacts alateral end portion of the field plate portion and a second dielectricconstant adjustment layer formed on the first dielectric constantadjustment layer, wherein a ratio of an electric charge of a carrierpulled out from the lateral end portion of the field plate portion ofthe first electrode through the inter-electrode insulator film and thefirst dielectric constant adjustment layer, to an electric charge of acarrier pulled out from a bottom surface of the first electrode is 65%or larger.
 12. The semiconductor device according to claim 11, whereinthe first dielectric constant adjustment layer includes at least one ofSiO₂, Si₃N₄, Al₂O₃, and MgO.
 13. The semiconductor device according toclaim 11, wherein the first dielectric constant adjustment layerincludes a polyimide film in which ceramic particles of at least one ofTiO₂, SiO₂, Al₂O₃, HfO, ZrO, HfSi_(x)O_(y), and ZrSi_(x)O_(y) are mixed.14. The semiconductor device according to claim 11, wherein the firstdielectric constant adjustment layer includes a polymeric materialincluding at least one metal of Cu, Ni, Ag, Al, Zn, Co, Fe, and Mn. 15.A semiconductor device, comprising: a first semiconductor layer that isformed on a substrate and formed of a nitride-based semiconductor; asecond semiconductor layer that is formed on a surface of the firstsemiconductor layer and formed of a nitride-based semiconductor having awider band-gap than the first semiconductor layer; a first electrodethat is formed on a surface of the second semiconductor layer; a secondelectrode that is formed on the surface of the second semiconductorlayer; an inter-electrode insulator film that is formed between thefirst electrode and the second electrode on the surface of the secondsemiconductor layer; and a dielectric constant adjustment layer that isformed on the inter-electrode insulator film and formed of an electricinsulator, wherein the first electrode has a field plate portion formedso as to ride on the inter-electrode insulator film, and the dielectricconstant adjustment layer has a first dielectric constant adjustmentlayer that contacts a lateral end portion of the field plate portion anda second dielectric constant adjustment layer formed on the firstdielectric constant adjustment layer, wherein the first dielectricconstant adjustment layer includes a silicone resin material or ahigh-dielectric polymeric material.
 16. The semiconductor deviceaccording to claim 15, wherein the first dielectric constant adjustmentlayer includes a silicone resin material.
 17. The semiconductor deviceaccording to claim 15, wherein the first dielectric constant adjustmentlayer includes a high-dielectric polymeric material.